Because of their excellent adhesion and rigidity properties, epoxy resins have been used extensively as matrix resins for composite materials. In particular, bisphenol A epoxy resins are used most commonly because of their good handling property and balanced physical properties but their use is limited to applications where high heat resistance is not required. Compositions chiefly composed of N,N,N',N'-tetraglycidyl diaminodiphenylmethane and 4,4'-diaminodiphenyl sulfone have been used extensively in applications where high heat resistance is required. In spite of their high heat resistance and rigidity, the use of these compositions is again limited by their low elongation and toughness. With a view to solving this problem, the present inventors previously proposed a novel improved epoxy resin composition in Unexamined Published Japanese Patent Application (JP-A) No. 63-10618.